Node unit capable of measuring delay and distributed antenna system including the same

ABSTRACT

A node unit of distributed antenna system, the node unit comprises a delay measuring part configured to transmit a first test signal for delay measurement to an upper adjacent node unit and detect the first test signal looped back via the upper adjacent node unit and measure a round trip delay between the node unit and the upper adjacent node unit, and a delay providing part disposed on a signal transmission path through which a second test signal for delay measurement, to be transmitted from a lower adjacent node unit, is to be looped back to the lower adjacent node unit, and configured to provide a delay corresponding to the round trip delay.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of International Application No.PCT/KR2014/013103, filed Dec. 31, 2014, and claims priority from KoreanPatent Application No. 10-2014-0194366 filed Dec. 30, 2014, the contentsof which are incorporated herein by reference in their entireties.

BACKGROUND

1. Field

The inventive concept relates to a transmission delay measurement andmore particularly, to a node unit capable of measuring a delay and adistributed antenna system including the same.

2. Description of Related Art

Delay equalization processing in a distributed remote device (e.g., aremote unit in a distributed antenna system, a remote radio head (RRH)in a base station distributed system, or the like) is required in adistributed transmission system for mobile communication signals, suchas the distributed antenna system or the base station distributedsystem.

In particular, the delay equalization processing is important in anorthogonal frequency division multiplexing (OFDM)-based signaltransmission system such as long term evolution (LTE) or WIBRO. InOFDM-based signals, it is important to maintain orthogonality betweencarrier wavers for performing a discrete Fourier transform (DFT)operation in a cell overlapping region. This is because, in order tomaintain the orthogonality, it is required to provide mobilecommunication services through distributed remote devices at the sametime.

Therefore, in a signal distributed transmission system, the accuratemeasurement of a transmission delay is required as a precondition of thedelay equalization processing for improving the quality of mobilecommunication services.

SUMMARY

An embodiment of the inventive concept is directed to a node unitcapable of measuring a delay and/or a distributed antenna systemincluding the same.

According to an aspect of the inventive concept, there is provided anode unit of distributed antenna system, the node unit comprising: adelay measuring part configured to transmit a first test signal fordelay measurement to an upper adjacent node unit and detect the firsttest signal looped back via the upper adjacent node unit and measure around trip delay between the node unit and the upper adjacent node unit;and a delay providing part disposed on a signal transmission paththrough which a second test signal for delay measurement, to betransmitted from a lower adjacent node unit, is to be looped back to thelower adjacent node unit, and configured to provide a delaycorresponding to the round trip delay.

According to an exemplary embodiment, the node unit may be a node unitconnected to a main unit of the distributed antenna system.

According to an exemplary embodiment, the node unit may be any oneremote unit among a plurality of first remote units connected to themain unit or a hub unit connected to the main unit to distribute themobile communication signals to a plurality of second remote unitsconnected the hub unit.

According to an exemplary embodiment, the node unit may further comprisea first framer/deframer for transmitting and receiving signals betweenthe node unit and the upper adjacent node unit, wherein the delaymeasuring part configured to detect the first test signal looped backfrom the upper adjacent node unit through the first framer/deframer.

According to an exemplary embodiment, the node unit may further comprisea second framer/deframer for transmitting and receiving signals betweenthe node unit and the lower adjacent node unit, wherein the delayproviding part configured to provide the delay to the second test signaltransmitted from the lower adjacent node unit through the secondframer/deframer.

According to an exemplary embodiment, the node unit may further comprisea control part configured to set a value of the delay in accordance witha value of the measured round trip delay.

According to an exemplary embodiment, the control part may receive adelay measurement start signal from the upper adjacent node unit, andcontrol measurement for the round trip delay to be started through thedelay measuring part in response to the received delay measurement startsignal.

According to an exemplary embodiment, after the measuring of the roundtrip delay through the delay measuring part and the providing of thedelay to the second test signal through the delay providing part arecompleted, the control part may transmit the received delay measurementstart signal to the lower adjacent node unit.

According to an exemplary embodiment, after the measuring of the roundtrip delay through the delay measuring part and the providing of thedelay to the second test signal through the delay providing part arecompleted, the control part may generate a delay measurement startsignal instructing the start of delay measurement at the lower adjacentnode unit, and transmit the generated delay measurement start signal tothe lower adjacent node unit.

According to another aspect of the inventive concept, there is provideda distributed antenna system including a main unit and a plurality ofnode units connected the main unit, wherein each of the plurality ofnode units may be implemented a node unit described above.

According to the inventive concept, in a signal transmission system inwhich communication node units for mobile communication services arebranch-connected to each other, it is possible to automatically measurea transmission delay in the signal transmission system.

Also, according to the inventive concept, delay compensation can beperformed by considering the measured transmission delay caused by thetransport medium in the signal transmission system. Thus, it is possibleto improve the quality of mobile communication services in the case ofOFDM-based signal transmission such as LTE or WIBRO, in which it isparticularly important to synchronize service times of mobilecommunication signals.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram illustrating an example of a topology of adistributed antenna system as a form of a signal distributedtransmission system to which the inventive concept is applicable.

FIG. 2 is a block diagram illustrating an embodiment of a remote unit inthe distributed antenna system to which the inventive concept isapplicable.

FIG. 3 is a diagram illustrating a transmission delay measuring methodof a related art compared with an embodiment of the inventive concept.

FIG. 4 is a block diagram illustrating a general signal transmissionpath between upper and lower node units, based on a specific node unitin the distributed antenna system to which the inventive concept isapplicable.

FIG. 5 is a diagram illustrating a node unit capable of measuring adelay and a distributed antenna system including the same according toan embodiment of the inventive concept.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary embodiments of the inventive concept will be described belowin more detail with reference to the accompanying drawings. Theinventive concept may, however, be embodied in different forms andshould not be construed as limited to the embodiments set forth herein.Rather, these embodiments are provided so that this disclosure will bethorough and complete, and will fully convey the scope of the inventiveconcept to those skilled in the art. Throughout the disclosure, likereference numerals refer to like parts throughout the various figuresand embodiments of the inventive concept.

In description of the inventive concept, detailed explanation of knownrelated functions and constitutions may be omitted to avoidunnecessarily obscuring the subject manner of the inventive concept.Ordinal numbers (e.g. first, second, etc.) are used for descriptiononly, assigned to the elements in no particular order, and shall by nomeans specify the name of the pertinent element or restrict the claims.

It will be understood that when an element is “connected” or “coupled”to another element, the element may be directly connected or coupled toanother element, and there may be an intervening element between theelement and another element. To the contrary, it will be understood thatwhen an element is “directly connected” or “directly coupled” to anotherelement, there is no intervening element between the element and anotherelement.

In the entire specification, when a certain portion “includes” a certaincomponent, this indicates that the other components are not excluded,but may be further included unless specially described. The terms“unit”, “-or/er” and “module” described in the specification indicate aunit for processing at least one function or operation, which may beimplemented by hardware, software and a combination thereof.

It is noted that the components of the inventive concept are categorizedbased on each main function that each component has. Namely, two or morethan two component units, which will be described below, may be combinedinto one component unit or one unit may be classified into two or morethan two component units for each function. Each of the component units,which will be described below, should be understood to additionallyperform part or all of the functions that another component has, inaddition to the main function that the component itself has, and inaddition, part of the functions that each component unit has may beexclusively performed by another component unit.

Hereinafter, exemplary embodiments of the inventive concept will bedescribed in detail with reference to the accompanying drawings.

Hereinafter, a distributed antenna system is applicable will be mainlydescribed as an application example to which a delay measuring methodaccording to an embodiment of the inventive concept. However, theembodiment of the inventive concept may be identically or similarlyapplied to another signal distributed transmission system such as a basestation distributed system in addition to the distributed antennasystem. Hereinafter, a remote unit disposed in the distributed antennasystem as a communication node unit to which the delay measuring methodaccording to the embodiment of the inventive concept is applied will bedescribed as an example, but a hub unit in the distributed antennasystem may also correspond to the communication node unit. Also, it willbe apparent that a remote radio head (RRH) in the base stationdistributed system may correspond to the communication node unit.

FIG. 1 is a diagram illustrating an example of a topology of adistributed antenna system as a form of a signal distributedtransmission system to which the inventive concept is applicable.

Referring to FIG. 1, the distributed antenna system (DAS) includes abase station interface unit (BIU) 10 and a main unit (MU) 20, whichconstitute a headend node unit of the DAS, a hub unit (HUB) 30 that isan extension node unit, and a plurality of remote units (RUs) 40respectively disposed at remote service positions. The DAS may beimplemented as an analog DAS or a digital DAS. When necessary, the DASmay be implemented as a hybrid of the analog DAS and the digital DAS(i.e., performance of analog processing on some node units and digitalprocessing on the other node units).

However, FIG. 1 illustrates an example of the topology of the DAS, andthe DAS may have various topologies in consideration of particularity ofits installation area and application field (e.g., in-building, subway,hospital, stadium, etc.). As such, the number of the BIU 10, the MU 20,the HUB 30, and the RUs 40 and connection relations between upper andlower node units may also be different from those of FIG. 1. In the DAS,the HUB 30 is used when the number of branches to be branched in a starstructure from the MU 20 is limited as compared with the number of RUs40 required to be installed. Therefore, the HUB 30 may be omitted whenonly the single MU 20 sufficiently covers the number of the RUs requiredto be installed, when a plurality of main units 20 are installed, when aplurality of MUs 20 are installed, or the like.

Hereinafter, each node unit and its function in the DAS applicable tothe inventive concept will be sequentially described based on thetopology of FIG. 1.

The BIU 10 may perform an interface function between a base stationtransceiver system (BTS) such as a base station and the main unit 20 inthe DAS. In FIG. 1, it is illustrated that a plurality of BTSs areconnected to a single BIU 10. However, the BIU 10 may be separatelyprovided for each provider, each frequency band, or each sector.

In general, a radio frequency (RF) signal transmitted to a BTS is asignal with high power. Therefore, the BIU 10 may convert the RF signalwith high power into a signal with power suitable to be processed in theMU 20 and transmit the converted signal to the MU 20. According to animplementation method, the BIU 10, as shown in FIG. 1, may receivemobile communication service signals for each frequency band (or eachprovider or each sector), combine the received signals, and thentransmit the combined signal to the MU 20.

When the BIU 10 converted a high-power signal from the BTS into alow-power signal, combines mobile communication service signals, andthen transmits the combined signal to the MU 20, the MU 20 distributesthe combined and transmitted mobile communication signal (hereinafter,referred to as ‘relay signal’) for each branch. In this case, when theDAS is implemented as the digital DAS, the BIU 10 may be separated intoa unit for converting a high-power RF signal into a low-power RF signal,and a unit for converting a low-power RF signal into an intermediatefrequency (IF) signal, performing digital signal processing on theconverted IF signal, and then combining the digital signal processedsignal. Alternatively, when the BIU 10 performs only the function ofconverting the high-power signal from the BTS into the low-power signal,the MU 20 may combine the transmitted relay signals and distribute thecombined relay signal for each branch.

As described above, the combined relay signal distributed from the MU 20is transmitted for each branch (see Branch #1, . . . , Branch #k, . . ., Branch #N of FIG. 1) through the HUB 20 or directly transmitted to theRUs 40. Each RU 40 separates the combined relay signal transmitted fromthe MU 20 for each frequency band and performs signal processing (analogsignal processing in the analog DAS and digital signal processing in thedigital DAS). Accordingly, each RU 40 transmits a relay signal to a userterminal in its own service coverage through a service antenna. Aspecific functional configuration of the RU 40 will be described indetail below with reference to FIG. 2.

In FIG. 1, it is illustrated that the BTS and the BIU 10 are connectedto each other through an RF cable, the BIU 10 and the MU 20 areconnected to each other through an RF cable, and all units from the MU20 to a lower node unit thereof are connected to each other throughoptical cables. However, a signal transport medium between node unitsmay be variously modified. As an example, the BIU 10 and the MU 20 maybe connected through an RF cable, but connected through an optical cableor a digital interface. As another example, the MU 20, HUB 30, and theRU 40 directly connected to the MU 20 may be connected to each otherthrough optical cables, and the cascade-connected RUs 40 may beconnected to each other through an RF cable, a twist cable, a UTP cable,etc. As still another example, the RU 40 directly connected to the MU 20may also connected to the MU 20 through an RF cable, a twist cable, aUTP cable, etc.

Hereinafter, this will be described based on FIG. 1. Therefore, in thisembodiment, each of the MU 20, the HUB 30, and the RU 40 may include anoptical transceiver module for transmitting/receiving optical signalsthrough electro-optic conversion/photoelectric conversion. Whenconnection between node units is made through a single optical cable,each of the MU 20, the HUB 30, and the RU 40 may include a wavelengthdivision multiplexing (WDM) element. This will be clearly understoodthrough functional description of the RU 20 in FIG. 2 to be describedlater.

The DAS may be connected, through a network, an external managementdevice (e.g., a network management server or system (NMS) of FIG. 1).Accordingly, a manager can remotely monitor a status and problem of eachnode unit in the DAS and remotely control an operation of each node unitthrough the NMS.

FIG. 2 is a block diagram illustrating an embodiment of an RU in the DASto which the inventive concept is applicable.

Here, the block diagram of FIG. 2 illustrates an implementation form ofthe RU 40 in the digital DAS in which connection between node units ismade through an optical cable. In addition, the block diagram of FIG. 2illustrates only components related to a function of providing servicesignals to a terminal in a service coverage through a forward path andprocessing terminal signals received from the terminal in the servicecoverage through a reverse path.

As described above, a node unit to which the delay measuring methodaccording to the embodiment of the inventive concept is applicable mayvary, including the hub unit (see reference numeral 30 of FIG. 1), theRRH in the base station distributed system, etc., in addition to theremote unit to be described later. Hereinafter, for convenience ofillustration, a remote unit in the DAS will be assumed and described.

Referring to FIG. 2, the RU 40, based on a downlink signal transmissionpath (i.e., a forward path), includes an optical to electrical (O/E)converter 50, a serializer/deserializer (SERDES) 44, a deframer 52, adigital signal processor (DSP) 70, a digital/analog converter (DAC) 54,an up converter 56, and a power amplification unit (PAU) 58.

Accordingly, in the forward path, an optical relay signaldigital-transmitted through an optical cable is converted into anelectrical signal (serial digital signal) by the O/E converter 50, theserial digital signal is converted into a parallel digital signal by theSERDES 44, and the parallel digital signal is reformatted by thedeframer 52 to be processed for each frequency band in the DSP 70. TheDSP 70 performs functions including digital signal processing on relaysignals for each frequency band, digital filtering, gain control,digital multiplexing, etc. The digital signal passing through the DSP 70is converted into an analog signal via the DAC 54 constituting the finalnode unit of a digital part 84. In this case, the analog signal is anIF, and hence frequency up-converted into an analog signal in theoriginal RF band through the up converter 56. The frequency up-convertedanalog signal (i.e., the RF signal) in the original RF band is amplifiedvia the PAU 58 to be transmitted through a service antenna (not shown).

The RU 40, based on an uplink signal transmission path (i.e., a reversepath), includes a low noise amplifier (LNA) 68, a down converter 66, ananalog/digital converter ADC 64, the DSP 70, a framer 62, the SERDES 44,and an electrical to optical (E/O) converter 60.

Accordingly, in the reverse path, an RF signal (i.e., a terminal signal)received through the service antenna (not shown) from a user terminal(not shown) in a service coverage is low-noise amplified by the LNA 68,the low-noise amplified signal is frequency down-converted into an IFsignal by the down converter 66, and the converted IF signal isconverted into a digital signal by the ADC 64 to be transmitted to theDSP 70. The digital signal passing through the DSP 70 is formatted in aformat suitable for digital transmission through the framer 62, theformatted digital signal is converted into a serial digital signal bythe SERDES 44, and the serial digital signal is converted into anoptical digital signal by the E/O converter 60 to be transmitted to anupper node unit through an optical cable.

Although not clearly shown in FIG. 2, in the state in which the RUs 40are cascade-connected to each other as shown in FIG. 1, the followingmethod may be used when a relay signal transmitted from an upper nodeunit among the RUs 40 is transmitted to a lower adjacent node unitcascade-connected to the upper node unit. For example, when an opticalrelay signal digital-transmitted from an upper node unit is transmittedto a lower adjacent node unit cascade-connected to the upper node unit,the optical relay signal digital-transmitted from the upper node unitmay be transmitted to the lower adjacent node unit in an order of theO/E converter 50→the SERDES 44→the deframer 52→the framer 62→the SERDES44→the E/O converter 60. This will be clearly understood through FIG. 4to be described later.

In FIG. 2, the SERDES 44, the deframer 52, the framer 62, and the DSP 70may be implemented as a field programmable gate array (FPGA). In FIG. 2,it is illustrated that the SERDES 44 and the DSP 70 are commonly used inthe downlink and uplink signal transmission paths. However, the SERDES44 and the DSP 70 may be provided for each path. In FIG. 2, it isillustrated that the O/E converter 50 and the E/O converter 60 areseparately provided. However, the O/E converter 50 and the E/O converter60 may be implemented as a single optical transceiver module (e.g., asingle small form factor pluggable (SFP) (see reference numeral 82 ofFIG. 2)).

In the above, a form of the topology of the DAS and an example of theconfiguration of the RU have been described. Particularly, the RU in thedigital DAS in which signals are digital-transmitted through thetransport medium has been mainly described in FIG. 2. However, the delaymeasuring method according to the embodiment of the inventive conceptmay be applied to an analog transmission system such as an analog DAS(i.e., a DAS in which signals are analog-transmitted through a transportmedium), as well as the digital DAS. Also, the delay measuring methodaccording to the embodiment of the inventive concept may be applied to acase where analog RF relays or digital RF relays are cascade-connectedto each other, as a case of signal distributed transmission. Inaddition, the delay measuring method according to the embodiment of theinventive concept may be applied to various cases where node units in amaster-slave relationship are connected to each other. Hereinafter, forconvenience of illustration, a case where the delay measuring methodaccording to the embodiment of the inventive concept is applied to RUsin the digital DAS will be mainly described.

Before detailed description of this (see FIG. 5), in order to help clearunderstanding of the delay measuring method according to the embodimentof the inventive concept, a conventional related art will be firstdescribed with reference to FIG. 3.

FIG. 3 is a diagram illustrating a transmission delay measuring methodof a related art compared with the embodiment of the inventive concept.

Referring to FIG. 3, as the transmission delay measuring methodaccording to the conventional art, there is generally used a method ofgenerating a test signal for delay measurement from a main unit,transmitting the generated test signal to a lower remote unitbranch-connected to the main unit, and detecting a loop-back pulselooped back via the lower remote unit, thereby measuring a transmissiondelay, based on a time delay between the test signal and the loop-backpulse (i.e., see a round trip delay of FIG. 3).

When a plurality of remote units cascade-connected to each other existon the same branch as the main unit, there is used a method oftransmitting a delay measurement signal from the main unit to the remoteunits and measuring each delay by using a pulse looped back via acorresponding remote unit.

If a delay from the main unit to each remote unit is measured by theabove-described method, the main unit transmits, to each remote unit, adelay compensation value necessary for delay compensation, so that thedelay compensation is made for each remote unit.

On the other hand, in the embodiment of the inventive concept, only themeasurement of a delay between a node unit and an upper adjacent nodeunit among a plurality of node units branch-connected to a main unit(i.e., a headend unit) is performed, so that delay measurement or/anddelay compensation is made. In the following description of FIG. 5, itwill be clearly understood that the method according to the embodimentof the inventive concept is distinguished from the method of FIG. 3.

FIG. 4 is a block diagram illustrating a general signal transmissionpath between upper and lower node units, based on a specific node unitin the distributed antenna system to which the inventive concept isapplicable.

In FIG. 2, the components related to path for transmitting or receivingsignals through the service antenna and their functions have beendescribed. On the other hand, components related to a path fortransmitting/receiving signals in a relationship with an upper node unitor transmitting/receiving signals in a relationship with a lower nodeunit and their functions are mainly described in FIG. 4.

In FIG. 4, it is assumed that the specific node unit is connected to theupper and lower node units through an optical cable. However, asdescribed in FIG. 1, the transport medium used in the connection betweenthe node units may vary. When the optical cable is not used, SFP #1 120and SFP #2 125 of FIG. 4 may be omitted. In FIG. 4, it is assumed thatsignals are digital transmitted through the transport medium, but thesignal transmission method between node units through the transportmedium is not limited thereto. When an analog transmission method isemployed rather than a digital transmission method, framers 140-1 and145-1, deframers 140-2 and 145-2, and SERDESs 130 and 135 may also beomitted, and replaced with other components for analog transmission.Hereinafter, this will be described based on FIG. 4. Here, a case wherethe node unit of FIG. 4 is a remote unit in the DAS will be described asan example (see FIG. 1).

In FIG. 4, forward path #1 is a signal transmission path through which amobile communication signal transmitted from an upper node unit of acorresponding remote unit is provided to a terminal in a servicecoverage through a service antenna (not shown). Thus, the forward path#1 of FIG. 4 is substantially identical to the forward path of FIG. 2.In the case of the forward path #1, a mobile communication signaldigital-transmitted from an upper node unit through a transport medium(in this example, an optical line) is optical-to-electrical convertedvia the SFP #1 120, the converted signal is converted into a paralleldigital signal via the SERDES #1 130, the converted digital signal isreformatted via the deframer #1 140-2, and the reformatted digitalsignal is input to a forward signal processing block 110. After thedigital signal transmitted from the forward signal processing block 110is processed, the processed digital signal is converted into an RFsignal in a frequency ban corresponding to each original mobilecommunication protocol, and the converted RF signal is transmitted tothe terminal in the service coverage through the service antenna (notshown).

In this case, when there exists an adjacent remote unit branch-connected(i.e., cascade-connected) to corresponding remote unit as a lower nodeunit thereof, the mobile communication signal digital-transmitted fromthe upper node unit may be transmitted to the lower node unit throughforward path #2 of FIG. 4. The forward path #2 is a signal transmissionpath through which the mobile communication signal digital-transmittedfrom the upper node unit is transmitted to the lower node unit throughthe transport medium via the SFP #1 120, the SERDES #1 130, the deframer#1 140-2, the framer #2 145-1, the SERDES #2 135, and the SFP # 125.

In FIG. 4, reverse path #1 is a signal transmission path through whichthe mobile communication signal received from the terminal in theservice coverage through the service antenna (not shown) of thecorresponding remote unit is transmitted to the upper node unit (finallytransmitted to a base station). Thus, the reverse path #1 of FIG. 4 issubstantially identical to the reverse path of FIG. 2. In the case ofthe reverse path #1, the mobile communication signal received throughthe service antenna (not shown) is subjected to low noise amplification,frequency down-conversion, digital conversion, digital signalprocessing, etc., and then input to a reverse signal combiner (Rxsummer) 150. However, when the corresponding remote unit is a branchtermination node unit, the digital signal passing through the reversesignal processing block 115 may be immediately input to the framer #1140-1.

The Rx summer 150 combines a digital signal input through the reversepath #1 and a digital signal input through reverse path #2. Here, thereverse path #2 of FIG. 4 is a signal transmission path through which,when another remote unit exists at the lower node unit of thecorresponding remote unit, a reverse digital signal transmitted from thelower node unit is transmitted. A mobile communication signal subjectedto optical digital transmission from the lower node unit isoptical-to-electrical converted by the SFP #2 125, and the convertedsignal is input to the Rx summer 150 via the SERDES #2 135 and thedeframer #2 145-2.

The reverse digital signal signal-combined by the Rx summer 150 istransmitted to the upper node unit through the transport medium via theframer #1 140-1, the SERDES #1 130, and the SFP #1 120. The reversedigital signal is finally transmitted to the base station.

Hereinafter, for convenience of illustration, the framer #1 140-2, thedeframer #1 140-2, the SERDES #1 130, and the SFP #1 120 of FIG. 4 arecomponents related to the signal transmission path of signals to bereceived from the upper node unit or transmitted to the upper node unit,and hence called “upper interface components.” Similarly, the framer #2145-1, the deframer #2 145-2, the SERDES #2 135, and the SFP #2 125 ofFIG. 4 are components related to the signal transmission path of signalsto be received from the lower node unit or transmitted to the lower nodeunit, and hence called “lower interface components.”

This will be clearly understood through description of FIG. 5 to bedescribed below. However, according to the case of digital opticaltransmission, the upper interface components (i.e., the framer #1 140-2,the deframer #1 140-2, the SERDES #1 130, and the SFP #1 120 of FIG. 4)participate in the signal transmission of a test signal to betransmitted from the corresponding node unit to the upper node unit andthen looped back for the purpose of delay measurement. Also, the lowerinterface components (i.e., the framer #2 145-1, the deframer #2 145-2,the SERDES #2 135, and the SFP #2 125 of FIG. 4) participate in thesignal transmission of a test signal to be transmitted from the lowernode unit to the corresponding node unit and then looped back for thepurpose of delay measurement.

FIG. 5 is a diagram illustrating a node unit capable of measuring adelay and a DAS including the same according to an embodiment of theinventive concept.

Hereinafter, for convenience of illustration, components related to thedelay measuring method according to the embodiment of the inventiveconcept will be first described based on RU #1 of FIG. 5, and the delaymeasuring method performed by totally considering lower node unitsbranch-connected to the RU #1 will be described.

In the delay measuring method according to the embodiment of theinventive concept, the RU #1 includes a delay measuring part 210, adelay providing part 220, and a control part 230. According to the caseof digital transmission through a transport medium, the delay measuringpart 210, the delay providing part 220, and the control part 230 may beimplemented in a digital part in the RU #1, and implemented in a singleFPGA. It will be apparently understood by those skilled in the art thatother modifications are possible.

In order to perform delay measuring method according to the embodimentof the inventive concept, the delay measuring part 210 transmits a testsignal for delay measurement to an adjacent node unit (here, an MUconstituting a headend unit) branch-connected to a corresponding nodeunit (here, the RU #1) through the transport medium.

In FIG. 5, it is illustrated that the test signal for delay measurementis a test pulse that is a single pulse. However, it will be apparentthat various modifications are possible. For example, the test signalmay be a test pulse having a specific bit pattern, and an encodedmodulation signal corresponding to a mobile communication signal(obtained by reproducing a mobile communication signal) to be actuallyserviced may be used as the test signal. For example, since service timesynchronization is important in an OFDM-based signal, a test signalobtained by reproducing a corresponding OFDM-based signal (e.g., LTE,WIBRO, etc.) in which delay measurement is required may be used as thetest signal. Also, the test signal may be used to measure a transmissiondelay caused by the transport medium for each corresponding servicefrequency band by being carried in a use frequency band of acorresponding mobile communication service and transmitted to an uppernode unit.

Here, the delay measuring part 210 functions to measure a transmissiondelay specialized for a transport medium (or/and interface components ona signal transmission path for performing signal transmission throughthe transport medium), such as a characteristic of the transport medium,an installation length of the transport medium, or an installation pathof the transport medium. In FIG. 5, the case of digital opticaltransmission is illustrated, and hence an SFP, a SERDES, a framer, and adeframer may correspond to the interface components for performingsignal transmission through the corresponding transport medium (in FIG.5, an optical cable) (see description of FIG. 4). Thus, a case where thedelay measuring part 210 is disposed at the rear of a framer/deframer140 constituting a termination of upper interface components isillustrated in FIG. 5. However, it will be apparent that the position ofthe delay measuring part 210 may be variously modified.

The test signal transmitted to the upper node unit (i.e., the MU) by thedelay measuring part 210 is looped back to the RU #1 via aframer/deframer of the upper node unit through the transport medium. Thedelay measuring part 210 detects a looped-back signal (hereinafter,referred to as a ‘loop-back signal’), to measure a transmission delaybetween the MU and the RU #1 (more clearly, a round trip delay).

If it is assumed that a round trip delay between the MU and the RU #1,measured in the RU #1, is delay A as illustrated in FIG. 5, thetransmission of a mobile communication signal between the MU and the RU#1 is performed in a single direction (i.e., a forward or reversedirection), and therefore, an actual signal transmission delay throughthe transport medium may be approximately ½ of the round trip delay.However, the method of calculating the actual transmission delay fromthe round trip delay is experimentally, statistically, andmathematically determined, including various additional factors providedby a system designer, and therefore, its detailed description will beomitted.

The delay measurement performed through the delay measuring part 210 maybe started according to a delay measurement start signal transmittedfrom the headend unit or the NMS of FIG. 1. For example, the controlpart 230 of the RU #1 may receive a delay measurement start signaltransmitted from the headend unit or the NMS, control the delaymeasuring part 210 such that delay measurement is started according tothe received delay measurement start signal. In this case, the delaymeasurement start signal may be transmitted through a control &management (C&M) channel or through a downlink data channel of themobile communication signal.

Also, the delay measurement start signal may be simultaneouslytransmitted to a plurality of node units connected on the same branch asthe headend unit (MU of FIG. 5). However, the delay measurement startsignal may be sequentially transmitted to the plurality of node units.For example, the delay measurement start signal may be sequentiallytransmitted in such a manner that the delay measurement start signal isfirst transmitted a node unit (i.e., a node unit constituting a startpoint in a corresponding branch) connected directly to the headend unit,transmitted to a next node unit, and then transmitted to a next nodeunit.

In this case, the sequential transmission of the delay measurement startsignal may be performed under direct control of the headend unit or theNMS. However, the sequential transmission of the delay measurement startsignal may be performed in such a manner that the node unit firstreceiving the delay measurement start signal completes theabove-described delay measuring process (more accurately, completes allprocesses up to a delay providing process performed by the delayproviding part 220, which will be described later) and then transmitsthe delay measurement start signal to a lower adjacent node unit.Alternatively, the transmission of the delay measurement start signalfrom the headend unit or the NMS may be performed on only a branch startnode unit, and after the delay measuring process in the correspondingnode unit is completed, a control part (see reference numeral 230 ofFIG. 5) of the corresponding node unit may generate a delay measurementstart signal and transmit the generated delay measurement start signalto the lower adjacent node unit.

In the above, it has been described that the delay measurement startsignal is first generated and then transmitted to the lower node unit.However, it will be apparent that other modifications are possible. Forexample, the delay measurement in a corresponding node unit may bepreviously set to start in a specific time zone. Alternatively, thedelay measurement in a corresponding node unit may be may be startedunder control of a field worker.

As described above, if a round trip delay (see delay A of FIG. 5 in thecase of the RU #1) caused by the transport medium in a relationshipbetween the corresponding node unit and the upper adjacent node unit ismeasured by the delay measuring part 210, the control part 230 sets adelay value such that a delay corresponding to the measured round tripdelay is provided by the delay providing part 220.

The delay providing part 220 is disposed on a signal transmission paththrough which a test signal for delay measurement, to be transmittedfrom a lower adjacent node unit (here, RU #2) branch-connected to thecorresponding node unit (here, the RU #1), is to be looped back to thelower adjacent node unit. That is, in FIG. 5, the delay providing part220, based on the corresponding loop-back path, is disposed at the rearof a framer/deframer 145 constituting a termination of the lowerinterface components branch-connected to the lower node unit through thetransport medium. In this case, it will be apparent that the position ofthe delay providing part 220 may be variously modified.

The delay providing part 220 is disposed on the signal transmission paththrough which the test signal is to be transmitted from the loweradjacent node unit and then looped back, so that a delay is forciblyprovided in the loop-back process of the test signal transmitted fromthe delay measuring part built in the lower adjacent node unit (here,the RU #2).

Referring to FIG. 5, the delay A that is the round trip delay throughthe transport medium between the MU and the RU #1, previously measuredby the delay measuring part 210, is forcibly provided to the delayproviding part 220 of the RU #1. Accordingly, although only the delaymeasurement is performed according to the loop-back signal transmissionpath through the transport medium between the RU #2 and the RU #1 thatis an adjacent upper node unit, it is possible to measure a round tripdelay (i.e., a round trip delay in the entire transport mediuminterposed between the RU #2 and the MU that is a headend unit, seedelay B of FIG. 5) to which the round trip delay (i.e., the delay A)caused by the transport medium between the MU and RU #1 is reflected.The round trip delay (i.e., the delay B) measured by a delay measuringpart of the RU #2 is applied by a delay providing part of thecorresponding node unit. The delay B is reflected to delay measurementin a lower node unit of the RU #2. This process is repeatedly performedon node units up to a node unit constituting a termination in the samebranch in the same manner.

Thus, in the delay measuring method according to the embodiment of theinventive concept, a round trip delay caused by a transport mediumbetween a corresponding node unit in node units branch-connected to eachother and an upper node unit directly connected (adjacent) thereto ismeasured, so that it is possible to measure the entire round trip delayfrom the corresponding node unit subjected to the delay measurement to aheadend unit. To this end, the above-described components fortransmission delay measurement, including the delay measuring part 210,the delay providing part 220, and the control part 230, may be built ineach node unit branch-connected to the headend unit.

While the inventive concept has been described with respect to thespecific embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the inventive concept as defined in thefollowing claims.

What is claimed is:
 1. A node unit of distributed antenna system, thenode unit comprising: a delay measuring part configured to transmit afirst test signal for delay measurement to an upper adjacent node unitand detect the first test signal looped back via the upper adjacent nodeunit and measure a round trip delay between the node unit and the upperadjacent node unit; and a delay providing part disposed on a signaltransmission path through which a second test signal for delaymeasurement, to be transmitted from a lower adjacent node unit, is to belooped back to the lower adjacent node unit, and configured to provide adelay corresponding to the round trip delay.
 2. The node unit of claim1, wherein the node unit is a node unit connected to a main unit of thedistributed antenna system.
 3. The node unit of claim 2, wherein thenode unit is any one remote unit among a plurality of first remote unitsconnected to the main unit or a hub unit connected to the main unit todistribute the mobile communication signals to a plurality of secondremote units connected the hub unit.
 4. The node unit of claim 2,further comprising a first framer/deframer for transmitting andreceiving signals between the node unit and the upper adjacent nodeunit, wherein the delay measuring part configured to detect the firsttest signal looped back from the upper adjacent node unit through thefirst framer/deframer.
 5. The node unit of claim 4, further comprising asecond framer/deframer for transmitting and receiving signals betweenthe node unit and the lower adjacent node unit, wherein the delayproviding part configured to provide the delay to the second test signaltransmitted from the lower adjacent node unit through the secondframer/deframer.
 6. The node unit of claim 2, further comprising acontrol part configured to set a value of the delay in accordance with avalue of the measured round trip delay.
 7. The node unit of claim 6,wherein the control part receives a delay measurement start signal fromthe upper adjacent node unit, and controls measurement for the roundtrip delay to be started through the delay measuring part in response tothe received delay measurement start signal.
 8. The node unit of claim7, wherein, after the measuring of the round trip delay through thedelay measuring part and the providing of the delay to the second testsignal through the delay providing part are completed, the control parttransmits the received delay measurement start signal to the loweradjacent node unit.
 9. The node unit of claim 6, wherein, after themeasuring of the round trip delay through the delay measuring part andthe providing of the delay to the second test signal through the delayproviding part are completed, the control part generates a delaymeasurement start signal instructing the start of delay measurement atthe lower adjacent node unit, and transmits the generated delaymeasurement start signal to the lower adjacent node unit.
 10. Adistributed antenna system comprising: a main unit; and a plurality ofnode units connected to the main unit, wherein each of the plurality ofnode units includes: a delay measuring part configured to transmit afirst test signal for delay measurement to an upper adjacent node unitand detect the first test signal looped back via the upper adjacent nodeunit and measure a round trip delay between the node unit and the upperadjacent node unit; and a delay providing part disposed on a signaltransmission path through which a second test signal for delaymeasurement, to be transmitted from a lower adjacent node unit, is to belooped back to the lower adjacent node unit, and configured to provide adelay corresponding to the round trip delay.
 11. The distributed antennasystem of claim 10, wherein each of the plurality of node units furtherincludes a first framer/deframer for transmitting and receiving signalsbetween the node unit and the upper adjacent node unit, wherein thedelay measuring part configured to detect the first test signal loopedback from the upper adjacent node unit through the firstframer/deframer.
 12. The distributed antenna system of claim 11, whereineach of the plurality of node units further includes a secondframer/deframer for transmitting and receiving signals between the nodeunit and the lower adjacent node unit, wherein the delay providing partconfigured to provide the delay to the second test signal transmittedfrom the lower adjacent node unit through the second framer/deframer.13. The distributed antenna system of claim 10, wherein each of theplurality of node units further includes a control part configured toset a value of the delay in accordance with a value of the measuredround trip delay.
 14. The distributed antenna system of claim 13,wherein the control part receives a delay measurement start signal fromthe upper adjacent node unit, and controls measurement for the roundtrip delay to be started through the delay measuring part in response tothe received delay measurement start signal.
 15. The distributed antennasystem of claim 14, wherein, after the measuring of the round trip delaythrough the delay measuring part and the providing of the delay to thesecond test signal through the delay providing part are completed, thecontrol part transmits the received delay measurement start signal tothe lower adjacent node unit.